On 1/5/2011 5:38 PM, Steve Hunt wrote:
Do some simple
> geometry and you'll see that at really low elevation angles - 5 degrees
> say - the ground reflections from an 80m quarter-wave vertical will
> extend out to at least 750ft from the base of the vertical. A typical
> ground radial system will do nothing about the conductivity there.
Whoa.... I would need a much bigger yard for those !
> You can check this using EZNECs ability to have two different ground
> media. Set up Media 1 as salt water extending for about 50ft around the
> vertical, and Media 2 as rocky soil beyond.
A local ham did that for me when I was researching
vertical antennas.... but I think I will re-visit the issue
with him and discover what might have gone wrong,
or where we might have leaped along a bit. He is the
sort of engineer who would be just as curious about
it as I am so he won't be offended or become defensive
if I challenge our previous work.
Look at the elevation
> patterns - they will be those which pertain to poor ground. Now
> gradually extend Media 1 outwards until you get the elevation pattern to
> match that over salt water - that will tell you how far out the
> reflections are taking place and how extensive your radial system would
> need to be.
Interesting point. This has not come up before, and
seems like a worthwhile distinction. I wonder how or
why this has been overlooked or not clarified in the
literature.
> Please note - I'm not saying a good ground system doesn't help. I'm
> saying that the usual ground radial system predominantly affects antenna
> efficiency and not elevation pattern.
>
> Please also note that nothing I have said conflicts with the Exam
> Question answers.
I see that. It does, I think, contradict what Old Gordo
says on his audio course. He seems to have conflated
the two concepts of efficiency and elevation - and given
the test questions, his comments about increasing low
angle radiation seem unnecessary, if not mistaken.
Hmmmm . . .
This is what Mr. Gordo says verbatim on Disc 6 of his Extra Class Exam
course:
"What strongly affects the shape of the far-field low angle
which we want? the low angle elevation pattern of a vertically
polarized antenna ? Well, that would be the conductivity
and dielectric constant of the soil in the area just below the
antenna. And the more conductive the soil is, the better
the low angle of radiation. How is the far-field elevation
pattern
of of a vertically polarized antenna affected by being mounted
over seawater versus, let's say, ah, dry rocky ground? Well the
low angle radiation increases and that is what we want is low
angle radiation.
Now don't read too much into this... when they say it increases,
it does not mean it goes up, it means that there is more signal
coming out of it at a low angle.
What is the main effect of placing a vertical antenna over an
imperfect ground... the big effect there is it will reduce the
low angle radiation and create high levels of radiation like
NVIS where you are bouncing signals and, as hams call
it, "warming the clouds."
So, if you are thinking of putting up a vertical antenna, on
your roof, then have plenty of ground radials, usually two
to three inch-wide copper foil strips, as the ground plane.
If you are thinking of doing a vertical antenna in your back
yard, relying on your soil as ground, unless you live right
on the water, I recommend don't. Put it up on a metal
shed or up on a roof, with ground foil radials, and you
will have a much better low angle radiation pattern."
So.... I guess I am re-thinking all of this and not so sure that Ol'
Gordo is sufficiently precise when he says all this about improving
vertical antenna low angle radiation. Or am I still missing something ?
Thanks.
=============== Happy Trails - James - K8JHR ============
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